The invention relates to induction motors, and particularly to a method and apparatus for controlling an induction motor.
Previous methods and apparatus of controlling an induction motor incorporate the changing of speed taps or the use of triac controls, which simply provide an effective reduction in voltage or flux so as to cause the motor to run at a reduced speed by the nature of slip. Slip is generally a measurement of how much the movement of the rotor follows the excitation field, and is defined as the difference between the frequency of the excitation energy and the speed of the motor. While these controls provide adequate speed control, they do so at the expense of efficiency as the motor runs at a higher slip which is proportional to rotor conduction loss.
U.S. Pat. No. 5,252,905 a shows and describes such a controller for a motor. The controller uses a single phase pulse width modulated inverter to control the speed of the motor at speeds that are less than the full operating speed of the motor. The voltage applied to the motor is adjusted in direct proportion to the frequency output from the pulse width modulator. That is, a constant voltage to frequency ratio is maintained. This constant voltage to frequency ratio results in a constant torque output regardless of the speed of the motor.